This invention relates to a flexible dam or weir. A prior art weir comprising an impermeable flexible membrane which is used in a river is shown in FIGS. 1-5. A body 1 is formed by a water-impermeable and flexible sheet, such as a rubber coated cloth. The body 1 is water-tightly secured at peripheral edges 5, 6 and 7 thereof to concrete bases 2, 3 and 4 which are formed on the bottom and both side banks of the river. Fluid such as water and air is introduced through inlet port 8 into the body 1 to thereby inflate the body 1 and form a dam or weir. In normal conditions, the water in the river is dammed up to a predetermined level depending on dam height and the excess water will flow thereover as shown in FIG. 2.
In higher level conditions of the river where the water greatly exceeds the height of the dam, the fluid within the body 1 is removed using port 8 to thereby empty and collapse the body 1 as shown in FIG. 3. The water then freely runs over the collapsed body 1. In this manner a collapsible dam is used.
In a situation where an alluvial formation exists (such as a delta) on the downstream side of the dam, for example, in an estuary as shown in FIG. 4, in case of a flood, the alluvial formation is eroded by the river water falling from the dam. However, in such a prior art type of dam, the erosive action of the river water against the alluvial formation downstream of the dam 9 may not be sufficient due to the dissapative effects of the dam action. As a result, the formation tends to harden since water does not flow over the entire delta area. As a result, a hardness may exist in the alluvial formation and viscosity in flow conditions of the alluvial formation is increased. During flood conditions substantial eroding will take place and when the dam is collapsed, it tends to fall on the remaining alluvial formation as shown in FIG. 5. This leads to the disadvantage that the cross section of the river is reduced to the dimension corresponding to a height H of the remaining alluvial material 12.